It is common practice to use integrally molded plastic disposables provided with a plurality of open-top wells sized to receive liquid samples for performing sample processing steps. Such disposables are commonly known as “microplates” or “multi-well plates”. Several variations of microplates are well-known in the art.
A sealing cover may be applied to the top surface of the microplate to air-tightly seal the wells containing the liquid samples for various reasons. One reason is the necessity to avoid evaporation of liquids in order to ensure the integrity of the samples contained. Another reason is to prevent spilling of the samples during transport of the microplate from one location to another. Yet another reason is to prevent cross contamination of individual samples so as to provide a generally sterile and controlled environment under which the processing steps can be carried out. Conventionally, sealing of the microplates is done prior to charging the microplates into a processing device for processing the liquid samples.
Especially in quantitative PCR (PCR=polymerase chain reaction), it is convenient to use transparent sealing covers such as thin plastic foils which allow for an optical detection of reaction products even during progress of reactions. In practical use, for instance, a plastic foil provided with an adhesive backing is placed on top of the microplate wherein the adhesive backing faces the microplate. The plastic foil is pressed on the microplate, e.g., by means of a pressure roll rolling back and forth to thereby obtain uniform adhesion of the sealing foil to the microplate. It is also known to use heat-activated adhesives which require large heated stamps to fix the foils. Adhesive foils, however, often cause problems with respect to an air-tight sealing of the wells which can result in an undesired evaporation of fluids, thus impairing the reproducibility of test results. Especially in the case of small volume samples, variation between various reaction mixtures may occur.
Better results can be obtained using thermally fusible plastic foils. In practical use, the foil is positioned on top of the microplate and heated in order to soften and melt the foil at the interface between the plate and foil. While heated, the sealing foil is pressed on the microplate to ensure a close adhesive fit with full contact to the microplate. Once cooled, the foil acts as a leak proof seal on top of the microplate. Conventionally, a single piece of foil is cut to size and placed over the microplate, e.g., spanned in a frame. The foil is then pressed on the microplate, e.g., by manually or automatically holding a heated sealing stamp down. In automated instruments, a driving mechanism is used to bring the sealing stamp in and out of contact with the foil. It is also known to use a reel of foil which is automatically cut into sections during operation normally used in situations where a number of microplates need to be sealed automatically.